1. Field of the Invention
The present invention pertains to the field of gyroscopic devices and more particularly concerns stable synchronous systems for driving the rotors of gyroscope spin motors.
2. Description of the Prior Art
Gyroscopes typically use hysteresis synchronous motors so that the rotor may be driven at a known constant speed. Such operation is desired where an output of the gyroscope is a precise calibrated output only for a given constant or known rate of rotation of the gyroscope rotor. However, the standard hysteresis synchronous motor has an inherent property of random synchronization. The seat of this property lies in the fact that each time the motor is started and subsequently synchronizes, the permanent magnetic poles of the hysteresis ring of the motor are reformed or relocated. Each time they are newly generated, their locations randomly shift and so do their magnitudes, all apparently under the influence of the revolving magnetomotive force in the rotor ring. Thus, the magnetic axis as established in the rotor in its last preceding operating period will normally not be in alignment with the revolving field when the motor is subsequently started, so that a new synchronizing point accordingly materializes. The foregoing random readjustment can induce undesired differences in the magnitude and phase of motor induced vibrations and can alter the magnetic coupling between the spin motor stator and the gyroscope rotor. The effects of the latter problems are of special concern for flexure-supported free rotor gyroscopes wherein the gyroscope rotor is articulated with respect to the spin motor stator. In precision gyrocompasses, they are also a special problem, since the accuracy of computation of navigational and surveying computers employed in association with the gyrocompass is also affected adversely by the same uncertainties.
It is understood that one attempt has been made, but with minor success, to overcome the foregoing problem by periodic high frequency interruption of the spin motor supply to use an averaging effect of many rapid random resynchronizations. The noise introduced by the method is detrimental to the achievement of low random drift; also, rotor slip caused by the periodic non-synchronous operation of the gyroscope rotor induces uncertainty in the rotor speed, an uncertainty that cannot be tolerated in calibrated inertial systems. Another suggested method was to use a periodic advance or retreat of the spin motor magnetic field which causes remagnetization of the hysteresis ring without the periodic rotor speed change caused by the power interruption technique. Although there appears to be a beneficial averaging feature also in this latter technique, it does not overcome rotor speed uncertainty and it is therefore not suited for use in precision applications.
These prior art concepts, while dealing generally with the problems of volatile poling of the rotor ring, do not in themselves solve the problems solved by the present invention, nor does the concept found in the H.K. Voigt U.S. patent application Ser. No. 20,758 for a "Stable Subsynchronous Drive System for Gyroscope Rotor", filed Mar. 15, 1979 and assigned to Sperry Corporation. The Voigt concept provides precise control over the rotor spin rate, the rotor being servo operated subsynchronously whereby a reference frequency generated by a spin frequency reference generator is synchronized with a precisely stable frequency standard, assuring the maintenance of constant rotor speed and permitting precisely calibrated gyroscope torquing. The gyroscope motor rotor is thus spun at a subsynchronous frequency so that the poles in the hysteresis ring are constantly rotating about the ring and their effect on average magnetic bias is averaged substantially to zero. Further, the drive motor is operated with good efficiency because its excitation is automatically and closely adjusted to be just sufficient to generate only the necessary motor torque for subsynchronous operation. While of value in many situations it is, however, of most interest where it is necessary for separate reasons to equip the gyroscope itself with a two-phase reference spin generator, a procedure not always practical in view of weight, size, and cost considerations.
Furthermore, the prior art in general does not adequately satisfy the need for a high efficiency synchronous rotor drive for field operation with low current drive from a readily transportable battery. To produce minimum battery drain requires that minimum energy be expended in bringing the gyroscope rotor up to its synchronous operating speed from stand-still and also that minimum power is used in steady state operation. The usual practice has not been found to be suitably efficient, since it is merely to apply a constant frequency, constant voltage level to the motor considerably greater in amplitude than the normal operating level until the rotor reaches its operating speed, and then to reduce the voltage level to a predetermined lower operating level.